This invention relates to apparatus for, and a method of, filtering solid cellulose-based material. In particular, the invention is concerned with preventing wastage of cellulosic material when fine particles, e.g. "pulp dust", of the cellulose-based material are created during shredding or comminuting of the cellulose-based material in sheet, roll or flock form. The invention also relates to forming a mixture containing cellulose dispersed in a solvent therefor, the mixture suitably forming a so-called "premix" prior to being subjected to heat and pressure to dissolve the cellulose in the solvent and to create a dope which can be subsequently shaped and extruded to form a cellulosic product, such as a fiber or filament.
McCorsley et al U.S. Pat. No. 4,211,574, McCorsley et al U.S. Pat. No. 4,142,913 and McCorsley U.S. Pat. No. 4,144,080 all disclose methods of making solid precursors of cellulose dispersed in amine oxide. In each of these known methods, cellulose is suspended in a mixture which is a non-solvent for the cellulose at the temperature of the suspension and which contains a tertiary amine oxide and water. The solid product which is produced is cellulose containing amine oxide and some water. On heating the solid product, which is suitably in the form of chips, the cellulose is dissolved to form a solution having a viscosity suitable for spinning. Although references are made to the cellulose being formed of comminuted wood pulp, there is no disclosure in these prior patents on how the wood pulp should be comminuted and whether any attempt is made to retrieve wood pulp dust created during the comminution of the wood pulp.
In McCorsley U.S. Pat. No. 4,416,698 in the preparation of a cellulose dope it is disclosed that, at elevated temperatures, cellulose dissolves rapidly and forms a solution of cellulose in tertiary amine N-oxide of more uniform composition when the tertiary amine N-oxide containing the preferred amount of water and the cellulose are ground to the same predetermined particle size and charged simultaneously to the barrel of an extruder. Typically the cellulose and tertiary amine N-oxide are ground in a mill through a 0.5 mm screen, the cellulose particle size being reduced without significant degradation of the cellulose molecular weight. In the extruder the mixture is heated to dissolve the cellulose in the tertiary amine N-oxide-water mixture to form a dope prior to extrusion to form a filament or film. However again no reference is made to retrieval of wood pulp dust created during the grinding of the cellulose.
In practice we have found that in order to produce high quality cellulose mixtures having a relatively high solids contents, it is desirable for cellulosic material, e.g. from rolls or sheets of wood pulp or wood pulp flock, to be shredded into relatively large flakes or particles, e.g. from 1 to 20 cm.sup.2, with the cellulosic material being compressed as little as possible at its cut edges. However we have found that typically, in the process of shredding and comminuting the cellulosic sheet material, a significant amount, e.g. up to about 2% by weight, of the cellulosic sheet material is converted into pulp dust which is conventionally lost to the process.